Update migration-islandora-workbench.md

[inikolaidis]

Purpose / why

Updating Workbench documentation page for clarity and links to Workbench official documentation.

What changes were made?

• Add introduction with links to repository and documentation
• Add Migration Strategies recording from IslandoraCon to page
• Add 3rd option for Paged Content
• Formatting fixes

Verification

Consulted Mark Jordan's Workbench official documentation here

Interested Parties

• @Islandora/documentation
• @Islandora/committers

Checklist

Pull-request Reviewer

Pull-request reviewer should ensure the following:

• Does this PR link to related issues?
• Does the proposed documentation align with the Islandora Documentation Style Guide?
• Are the changes accurate, useful, free of typos, etc?

Person Merging

The person merging should ensure the following:

• Does mkdocs still build successfully? (This is indicated by TravisCI passing. To test locally, and see warnings, see How To Build Documentation.)
• If pages are renamed or removed, have all internal links to those pages been fixed?
• If pages are added, have they been linked to or placed in the menu?
• Did the PR receive at least one approval from a committer, and all issues raised have been addressed?

[seth-shaw-asu]

Feel free to overrule my nitpick if you object.